The effects of Bacillus-Calmette Guerin (BCG) administration upon lymphocyte traffic was studied in BALB/cJ mice. Intravenous or subcutaneous injection of BCG resulted in increased localization (trapping) of Cr51-labelled syngeneic lymphocytes in the spleen or draining nodes. BCG-induced splenic trapping was biphasic, occurring 1-2 days after BCG injection, and reappearing 3 weeks later. Trapping in draining lymph nodes persisted and increased during the 24 days of study. These results are in contrast to those obtained with SRBC, where trapping was evident only within the first few days of antigen injection. Additional studies revealed that prior injection of BCG influenced the ensuing magnitude of SRBC-induced trapping. Intravenous administration of BCG suppressed SRBC-induced splenic trapping; both intravenous and subcutaneous injection of BCG enhanced trapping to SRBC in the draining lymph nodes. These studies suggest that one mechanism for potentiation of immune responses by BCG may operate via BCG's effects upon regulation of lymphocyte traffic to the lymphoid tissues.